The present invention generally relates to air fuel ratio control systems, and more particularly to an air fuel ratio control system suitable for reducing energy consumption in the combustion of a furnace.
A conventional air fuel ratio control system used in a combustion control system of a gas furnace is designed to maintain a constant air fuel ratio (a mixing ratio of air and fuel gas) by controlling a valve opening degree (a degree to which a valve is opened) of a control valve provided in a gas supplying pipe and a valve opening degree of a control valve provided in an air supplying pipe to become constant.
The relationship between a valve opening degree of a control valve and a flow rate through the control valve is not perfectly proportional to each other, and has non-linearity and hysteresis characteristics. Further, the control valves used in the above mentioned air fuel ratio control system are relatively large and have a large hysteresis. Therefore, it is difficult to control the air fuel ratio constant even when the valve-opening degree of each control valve is maintained constant. For this reason, the control valves are controlled to obtain such an air fuel ratio that the flow rate of air is slightly in excess with respect to the flow rate of fuel gas compared to a preset air fuel ratio in order to avoid incomplete combustion even when the air fuel ratio changes in a direction in which the quantity of air decreases. In other words, when an air ratio is described by a ratio of the actual flow rate of air and the theoretical flow rate of air, the control valves are controlled so that the air ratio assumes a valve in a range of approximately 1.2-1.4. Thus, the combustion system is operated with such an air fuel ratio that the flow rate of air is slightly in excess even when the flow rate of air decreases in a fluctuating range. However, the thermal efficiency of the furnace is inevitably decreased due to the excess air supplied to the furnace, and problems are introduced when attempts are made to reduce the energy consumption of the furnace.
In addition, according to some furnaces, it is preferable to set the air fuel ratio to an air fuel ratio in which the flow rate of air is slightly in excess or deficient compared to the air fuel ratio for the ideal combustion efficiency, as the load of the furnace decreases. For example, the fuel and the air velocities in the burner decreases as the load of the furnace decreases. As a result, an incomplete mixing tends to occur when the load of the furnace is small. For this reason, in order to ensure complete combustion, the air fuel ratio must be controlled so that the flow rate of air is slightly in excess as the load of the furnace decreases. In the case of a furnace which is not sufficiently air-tight, such as a continuous furnace, the pressure within the furnace deviates to the negative pressure side as the load of the furnace decreases. In this case, even when the fuel and air are supplied from the burner, with a constant air fuel ratio the air in the furnace is extremely in excess in most cases due to the air which enters from the outside. In this case, it is desirable in some furnaces to control the air fuel ratio on the burner side to obtain a air fuel ratio in which the flow rate of fuel becomes in excess as the load decreases, so that overall air fuel ratio within the furnace assumes an appropriate value with regard to the combustion efficiency.
In the conventional air fuel ratio control systems, however, it is impossible to control the air fuel ratio in the manner described above. Therefore, the conventional air fuel ratio control systems suffer problems in that the systems cannot be generally applied to furnaces having different requirements.